Pancreatic protein and bicarbonate outputs were measured in dogs with pancreatic fistulas receiving intravenous secretin while individual L-amino acids or mixtures of L-amino acids were perfused into proximal intestine, Both phenylalanine and tryptophan were potent stimuli of pancreatic secretion; alanine, leucine, and valine increased protein outputs but their effects were small and not statistically significant. Six other amino acids were ineffective. Tryptophan perfused together with phenylalanine augmented responses to phenylalanine; methionine, ineffective when perfused alone, did not alter responses to phenylalanine when perfused with the latter. Responses to phenylalanine perfused at concentrations above 8 mM were dependent on load and were mediated by lengths of proximal bowel greater than 10 cm.